This invention relates to arrangements and methods for digital data services to user terminals in a telecommunications network.
In today""s telecommunications networks there is a growing requirement for transmission of bursty, packet oriented data traffic such as that which arises when desktop computers are linked together using Internet Protocol. Often it is advantageous to provide the facility to mix packet data traffic, which tends to be bursty in nature, with other traffic such as digitised voice circuits, which require constant and regular bandwidth, on a single transmission bearer. While this facility is now becoming available in transport networks, it is not generally available in access networks between the premises of the end customers of a telecommunications network provider and the nearest point of presence in the operator""s network. Known technologies such as Asymmetric Digital Subscriber Line (ADSL) and Very high speed Digital Subscriber Line (VDSL) operate over standard telephony subscriber loop or twisted pair customer connections and offer bit rates from 1.5 Mbit/s to 25 Mbit/s, depending on the technology used and the distance to be covered. In such systems, the loop may carry both the digital data and analogue voice traffic, the two services being separated by splitters or filters. Those customers having a high speed digital access connection such as VDSL could benefit from a more flexible use of the digital link.
An object of the invention is to minimise or to overcome the above disadvantage.
A further object of the invention is to provide an improved arrangement and method for providing voice and data access to a user terminal over a subscriber loop.
According to a first aspect of the invention there is provided a method of multiplexing one or more plesiochronous packet data channels together with lower priority asynchronous traffic into a single composite data stream for transmission over a transmission link, said plesiochronous data packets each comprising a number of bytes together with a header element containing channel identification information and a packet length indicator; the method comprising assigning to each said plesiochronous data packet a transmission time such that the desired nominal data rate for the plesiochronous channel is achieved when packets containing the preferred number of bytes are transmitted at the preferred time intervals.
The method provides a means whereby one plesiochronous data stream or a plurality of plesiochronous data streams having the same or differing nominal bit rates can be multiplexed together with bursty packet data streams onto a single transmission bearer.
It will be understood that a synchronous data stream, where the bit rate bears a known, exact and constant relationship to a local clock signal source, is a special case of a plesiochronous stream, where the bit rate tolerance is zero. Thus the methods an d systems described here can be equally applied where one or more of the component streams is synchronous with respect to a local clock signal.
Groups of contiguous bytes from an input stream are encapsulated into a data packet or mini-cell. Mini-cells from individual input streams are multiplexed together according to a set of rules which determine how many contiguous bytes from an input stream are collected into a single mini-cell and in which order the mini-cells from separate input data streams are multiplexed together. Each mini-cell carries header information which conveys the channel identity and specifies the length of each mini-cell, and carries delineation information. The information contained in mini-cell headers allows the original data streams to be reconstructed at the receiving demultiplexer.
In a preferred implementation of this invention, the data packets used for the multiplexed transmission data stream are mini-cells according to ITU draft recommendation I.363.2 operating without encapsulation in ATM cells.
According to a further aspect of the invention, there is provided apparatus for multiplexing one or more plesiochronous packet data channels together with lower priority asynchronous traffic into a single composite data stream for transmission over a transmission link, said plesiochronous data packets each comprising a number of bytes together with a header element containing channel identification information and a packet length indicator; the apparatus comprising means for assigning to each said plesiochronous data packet a transmission time such that the desired nominal data rate for the plesiochronous channel is achieved when packets containing the preferred number of bytes are transmitted at the preferred time intervals
In a further embodiment, there is included in the mini-cells generated from plesiochronous data streams a parameter from which the position in the mini-cell of the frame boundary (for structured streams) or an artificially established frame boundary (for non structured streams) can be deduced. This parameter information is then used by the demultiplexing equipment to verify that the correct number of plesiochronous data bytes have been received and that no plesiochronous mini-cells have been lost due to errors on the transmission medium. In yet a further embodiment, there is included periodically in the multiplexed transmission stream a special delineation mini-cell. The payload of this delineation mini-cell contains a known or predictable bit pattern. In the event of loss of mini-cell delineation at the receiver, the receiver will search for a three byte sequence which passes the header error check test. Should the channel identifier (CID) derived from this supposed header match the CID of a delineation mini-cell and the length indicator correspond to the length of a delineation mini-cell then the receiver will check that the supposed payload also matches the expected payload of a delineation mini-cell. If all these criteria are fulfilled then there is a high degree of confidence that the true mini-cell boundaries have been located.
Advantageously, the multiplexer may impose a frame structure on the output data stream by periodically inserting a frame marker in the multiplexed output stream at a known time measured with respect to the clock frequency of the multiplexed data stream. This frame marker consists of a bit pattern which is identifiable at the receiving demultiplexer and a pointer from which the position of the start of the next mini-cell can be deduced in the event of loss of mini-cell delineation at the receiver.
In addition to carrying bursty packet oriented user data, a multiplexed bursty transmission capability could also be used to carry an embedded operations channel (EOC) for monitoring and control purposes.
The invention thus provides a flexible, robust and simple system whereby asynchronous bursty packet data and fixed bit rate so called plesiochronous data can be multiplexed onto a single transmission system.
A plesiochronous data stream is defined herein as a one bit serial transmission signal whose actual clock frequency when measured over a long time interval may depart from its nominal value by a small amount. Such data streams are extensively used in telecommunications networks. An example of such a stream is a 2048 kbit/s transmission stream according to the International Telecommunications Union (ITU) recommendation G.703. According to the recommendation, signals with a nominal data rate of 2048 kbit/s may depart from their nominal frequency by up to 50 parts per million.
In some applications, telecommunications service providers offer a plesiochronous service connection where the content of the data stream is determined entirely by the customer. The United Kingdom telecommunications operator BT provides such a service under the name xe2x80x98Megastreamxe2x80x99. An example application of such a connection would be as a link between the data networks on two geographically separate sites of a single company.
In other applications, the content of the data stream is structured in a known way. Many telecommunications services use a structure based on frames of 125 ms length, such as defined in ITU recommendation G.704. At the nominal rate of 2048 kbit/s, such a frame contains 32 channels of eight bits (one byte) each. According to G.704, the first byte in alternate frames contains, amongst others, a known bit pattern which can be used to achieve frame alignment, allowing the remainder of the byte oriented information to be extracted. Conventionally, a structured signal running nominally at 2048 kbit/s contains, in addition to the framing channel, 30 end user channels plus one signalling channel, each having a nominal capacity of 64 kbit/s. An example application of such a structured connection is to link a Private Branch Exchange (PBX) on a business customer""s premises to the telecommunication network provider""s local switching facility.
In some circumstances, not all end user channels within a structured plesiochronous data stream will be used. For example, a business customer may not immediately require say 30 external telephone lines, yet for convenience the network provider is likely to install a standard connection. A transmission scheme which allows such unused capacity to be reused for other traffic could then offer financial and operational advantages.
To achieve economy of transmission links it is often desirable to multiplex a number of component streams together onto a single link running at a correspondingly higher bit rate. Where the component streams are plesiochronous signals, the multiplexing equipment must adapt each stream to the clock rate of the multiplexed link. ITU recommendation G.742 specifies a method for adapting four plesiochronous 2048 kbit/s streams into a single stream of 8448 kbit/s. Where a component stream is running at an actual clock rate higher than the nominal rate, opportunities are provided in the multiplex frame structure to transmit periodically the resulting extra information, together with a control indicator to signal that extra information has been inserted. Conversely, if a component stream is running more slowly than its nominal rate, opportunities are provided in the multiplex frame structure to drop information and signal that information has been dropped. The demultiplexing equipment can then examine the control signals and reconstruct the original component streams at their original bit rates without loss or gain of information.
Such multiplexing schemes are widely used in the telecommunications industry, but are relatively inflexible in application. In practice, a company might require, for example, a structured partially filled 2048 kbit/s connection from the site PBX to the local telephone operator""s switch, an unstructured 2048 kbit/s connection to another site to carry legacy data services, and a number of asynchronous bursty packet data connections to be routed by a local telecommunications operator to an Internet Service Provider and a number of other company sites. Furthermore, the balance of communication requirements might change on a time of day or time of week basis when the requirement for voice circuits is reduced overnight and at weekends. A more flexible multiplexing scheme as envisaged herein could offer both an improved service and reduced costs to both the operator and the operator""s customer.
In preferred embodiment of the invention, traffic over the subscriber loop is carried in mini-cells such as defined for Asynchronous Transfer Mode (ATM) Adaptation Layer 2 (AAL2) in ITU draft recommendation I.363.2. Mini-cells comprise a header section having a fixed length of three bytes and a payload section having a variable length of up to 64 bytes. The header is transmitted first with the payload following immediately. The header contains, amongst others, fields specifying the identity of the attached payload channel (CID) and an indication of the length of the payload (LI). The CID and LI fields are 8 bits and 6 bits, respectively. Additionally, the header contains an error check field (HEC) of length 5 bits from which errors which may occur in the header during transmission of a mini-cell stream over an error-prone transmission link can be detected at the receiving node.
Initially, it was intended that AAL2 mini-cells should be transmitted within the payload of ATM cells. However, it is known that mini-cells can also be transmitted without using this encapsulation mechanism. In such a transmission system, the receiving entity must be able to identify the boundaries of the received mini-cells in order to extract correctly the transmitted data. For example, this may be done by hunting for a three byte sequence which, when submitted to the header error check process, registers no errors. This three byte sequence is then either a valid mini-cell header or another group of data bytes which happen to pass the error check. By extracting the payload length indicator (LI) from this supposed mini-cell header, a prediction of the location in the received byte stream of the next mini-cell header can be made. If this group of three bytes also passes the error check, then confidence is increased that the true mini-cell boundaries have been located. Further checks are made at subsequent predicted mini-cell header positions until confidence is increased to any level desired by the system designer. The process of identifying the mini-cell boundaries is known as delineation.